It happened with my Sony 2015 x8505c in 2017.
Blinking 3 red lights
Tv not powering on.
Broken mainboard they said.
Part was not available anymore to fix it :-(
I got a new x9305e 2017 model with discount because was just over 2 year warranty.
Contact your seller. The store you bought it from.
What model do you have?
Still warranty?